We have previously shown that a sequence more than 200 base pairs long is involved in the specific recognition of the phage attachment site of bacteriophage lambda. We have now generated shortened bacterial att DNA and cloned these DNA fragments. By characterizing such DNA we found that a DNA molecule carrying the 15 base pairs long common core sequence retained the capacity to act as the bacterial attachment site (att B). However, these DNA molecules were less active than molecules that carried two additional 4 base pair sequences of the original DNA on both sides of the common core. The nature of the cross-over point within the common core sequence during the integrative recombination reaction has also been investigated. A new method was developed to approach this problem, using the disintegration of 32P atoms incorporated in a DNA molecule to generate base-specific cleavage within a segment of DNA that derives from one of the parents of the reaction. Integrative recombination was shown to involve staggered cuts at unique sites within the common core sequence.